•Friction mechanisms were reviewed and compared between a FPE and CSE.•Sub-models were developed to describe friction force from friction mechanisms.•FPE doesn’t show advantage on piston ring ...friction force over the CSE.•Total friction loss of the FPE is nearly half of the CSE.
Friction work in free-piston engines is expected to be lower than in crankshaft engines due to the elimination of the crank mechanism. In this paper, friction mechanisms were reviewed and compared between a free-piston and crankshaft engine of similar size. The main friction mechanisms were identified to be the piston assembly including piston rings and piston skirt, valve train system, the crank and bearing system for the CSE, and the linear electric generator for the FPE. The frictional loss of each friction mechanism was estimated and discussed. A Stribeck diagram was used to simulate the piston ring friction during hydrodynamic lubrication, mixed lubrication, and boundary condition. It is found that the FPE doesn’t show advantage on piston ring friction force over the CSE, and the frictional loss from the piston ring is even higher. While the elimination of the crankshaft system reduces the frictional loss of the FPE, and the total friction loss of the FPE is nearly half of the CSE.
•A FPE model operated in two thermodynamic cycles is presented.•The engine performance for both gas exchange cycles are described.•Power distribution with different operation parameters are ...provided.•Advantages and disadvantages for the two thermodynamic cycles are summarised.
Free-piston engines (FPEs) are known to have a greater thermal efficiency (40–50%) than an equivalent and more conventional four-stroke reciprocating engines (30–40%). Modern FPEs are proposed for the generation of electric and hydraulic power, with a potential application in hybrid electric vehicles. The numerous FPE configurations considered to date have almost exclusively operated using a two-stroke thermodynamic cycle to improve the thermal efficiency, however it is well known that the application of two-stoke cycles can be limited by noise and exhaust gas emissions constraints. In this article, a numerical model is used to investigate the techno-feasibility of operating Newcastle University’s FPE prototype using a two- or four-stroke thermodynamic cycle. If operated as a four-stroke cycle, the linear generator must be used as both a motor and a generator resulting in a more irregular piston motion compared to corresponding operating in a two-stroke cycle. In four-stroke cycles, almost half the indicated power is consumed in overcoming the pumping losses of the motoring process. Whilst the heat release process is appears to be closer to a constant volume process when operated on two-stroke engine cycle, the peak cylinder pressure and compression ratio proved lower. In addition, a narrower power range is reported for a four-stroke cycle despite a corresponding higher thermal efficiency.
•The global control structure for a FPEG prototype is presented.•A Cascade control strategy is proposed for the piston stable operation level.•TDC of the previous stroke and velocity of the current ...stroke are taken for feedback.•Controller performance is improved on control delay, peak error and settling time.
The free-piston engine generator (FPEG) is a linear energy conversion system, which is known to have greater thermal efficiency than an equivalent and more conventional reciprocating engine. The piston motion of a FPEG is not restricted by a crankshaft-connection rod mechanism, it must be controlled to overcome challenges in the starting process, risk of misfire, and unstable operation. In this paper, the global control structure for a FPEG prototype is presented. A Cascade control strategy is proposed for the piston stable operation level, and PID controllers are used for both of the outer loop and inner loop. The measured top dead centre of the previous stroke and the piston velocity during the current stroke are taken for controller feedback, and the injected fuel mass is used as the control variable. The proposed cascade control implemented in the FPEG is shown to have good performance, the piston returns to a stable state in 0.5s. Compared with a single loop control strategy, the performance of cascade control is improved in terms of the control delay, peak error and settling time.
•Detailed friction models of each component in FPEG and TCE are presented.•Dynamic and friction characteristics of FPEG and TCE are compared and analyzed.•Effects of lubricating oil viscosity and ...temperature on friction loss are studied.•Total mean friction power in FPEG is obviously lower than that in TCE.
Compared with traditional crankshaft engines (TCEs), free-piston engine generators (FPEGs) present many potential advantages due to the abandonment of crankshaft mechanism. The friction loss in FPEGs is considered to be lower than that in TCE. However, few studies have calculated and analyzed the friction differences between them in any great detail. This paper presents an equivalent design of two-stroke TCE based on an existing FPEG prototype size, and then determines the main friction components of their prototypes. The comprehensive simulation models of FPEG and TCE are established in MATLAB/Simulink. Following this, the friction models and lubrication characteristics of each part in FPEG and TCE are described and compared based on the fundamental lubrication theory. The simulation results show good agreement with the experimental data for both the cold start and combustion power generation. And the overall error between them is less than 5%, indicating that the simulation model is reliable and can predict the prototype performance. Moreover, the effects of lubricating oil grade, temperature and operating frequency on the friction performance of FPEG are also investigated. Results show that the piston assembly friction power is less powerful in FPEG (118.3 W) than that in TCE (142.1 W), while the friction power of piston rings is slightly more powerful in FPEG than TCE. The total mean friction power is significantly lower in FPEG than that in TCE, with the proportion of total mean friction power to the indicated power being 11% in TCE and 6.7% in FPEG. This distribution of friction power indicates that when concerning friction loss, FPEG has an advantage over TCE. The piston ring friction power increases with an increase in lubricating oil temperature and operating frequency, and decreases with an increase in lubricating oil viscosity. The proportion of mean friction power to the indicated power shows a decreasing trend as the viscosity and operating frequency increase, and an increasing trend as the lubricating oil temperature increases.
•Starting strategies of free-piston engine generator by linear electric machine is investigated.•A novel method to start the engine by mechanical resonance is proposed.•A detailed comparison of the ...implementation of open-loop and close-loop strategies.•A detailed discussion of how the motor force can influence the starting process and the following combustion process.
Two general methods to start the engine are investigated by the linear electric machine operating as a linear motor and identified one for a specific FPEG prototype configuration. Based on that, a novel method to start the engine by mechanical resonance is proposed. Both simulation and test results are presented, and the numerical model is successfully validated. The results show that with a fixed motor force of 110N, within 0.5s, the maximum in-cylinder pressure could reach 13bar, indicating that the engine is ready for ignition. Further investigation on the engine performance with the closed-loop control strategy is carried out. The results demonstrate that it is feasible to start the FPEG with mechanical resonance with a closed-loop controlled electric linear machine that applies a proper constant motor force in the direction of the natural bouncing motion. With different starting motor force, the top dead centre (TDC) value for both cylinder is different during the first few running cycles, but the difference reduces and tends to be zero during the stable resonance state. There is not any significant difference observed on the engine frequency and piston profile during combustion process.
•The intermediate process of free-piston linear generator is investigated for the first time.•“Gradually switching strategy” is the best strategy in the intermediate process.•Switching at the top ...dead center position timing has the least influences on free-piston linear generator.•After the intermediate process, the operation parameters value is smaller than those before the intermediate process.
The free-piston linear generator (FPLG) has more merits than the traditional reciprocating engines (TRE), and has been under extensive investigation. Researchers mainly investigated on the starting process and the stable generating process of FPLG, while there has not been any report on the intermediate process from the engine cold start-up to stable operation process. Therefore, this paper investigated the intermediate process of the FPLG in terms of switching strategy and switching position based on simulation results and test results. Results showed that when the motor force of the linear electric machine (LEM) declined gradually from 100% to 0% with an interval of 50%, and then to a resistance force in the opposite direction of piston velocity (generator mode), the operation parameters of the FPLG showed minimal changes. Meanwhile, the engine operated more smoothly when the LEM switched its working mode from a motor to a generator at the piston dead center, compared with that at the middle stroke or a random switching time. More importantly, after the intermediate process, the operation parameters of FPLG were smaller than that before the intermediate process. As a result, a gradual motor/generator switching strategy was recommended and the LEM was suggested to switch its working mode when the piston arrived its dead center in order to achieve smooth engine operation.
Temperature rise of the tubular permanent magnet linear generator (TPMLG) might lead to insulation failure and demagnetization of permanent magnets, affecting the safe and stable operation of other ...equipment and the entire system. Herein, a bidirectional electromagnetic–thermal coupling method for analyzing the electromagnetic loss and thermal characteristics of a TPMLG considering the effect of increased temperature on the permanent magnet was proposed. To study the electromagnetic–thermal characteristics of the TPMLG under stable power generation, a two-dimensional electromagnetic field model and a three-dimensional temperature field model were established and coupled. The temperature field of the TPMLG was numerically calculated using computational fluid dynamics over finite volume method under natural air cooling and forced air cooling conditions. Effects of loss and air flow velocity on the steady temperature field were investigated. Results indicated that copper loss increased by 24.5% considering the influence of temperature rise. The windings’ top central position in the TPMLG was the spot with the highest temperature of 127.8 °C and there was a potential demagnetization risk for the permanent magnets. Some reference for future research of clarifying thermal characteristics and cooling design was provided.
•The heavy duty (HD) vehicle and freight sector needs to be decarbonised.•There are multiple technological pathways to achieving this.•Technology roadmaps offer some insight into technology will ...develop.•This paper uses these roadmaps to estimate how to decarbonise the HD sector.•We explore the impact on size, weight, CO2 emissions and energy consumptions.
Minimising tailpipe emissions and the decarbonisation of transport in a cost effective way remains a major objective for policymakers and vehicle manufacturers. Current trends are rapidly evolving but appear to be moving towards solutions in which vehicles which are increasingly electrified. As a result we will see a greater linkage between the wider energy system and the transportation sector resulting in a more complex and mutual dependency. At the same time, major investments into technological innovation across both sectors are yielding rapid advancements into on-board energy storage and more compact/lightweight on-board electricity generators.
In the absence of sufficient technical data on such technology, holistic evaluations of the future transportation sector and its energy sources have not considered the impact of a new generation of innovation in propulsion technologies. In this paper, the potential impact of a number of novel powertrain technologies are evaluated and presented. The analysis considers heavy duty vehicles with conventional reciprocating engines powered by diesel and hydrogen, hybrid and battery electric vehicles and vehicles powered by hydrogen fuel cells, and free-piston engine generators (FPEGs). The benefits are compared for each technology to meet the expectations of representative medium and heavy-duty vehicle drivers. Analysis is presented in terms of vehicle type, vehicle duty cycle, fuel economy, greenhouse gas (GHG) emissions, impact on the vehicle etc..
The work shows that the underpinning energy vector and its primary energy source are the most significant factor for reducing primary energy consumption and net CO2 emissions. Indeed, while an HGV with a BEV powertrain offers no direct tailpipe emissions, it produces significantly worse lifecycle CO2 emissions than a conventional diesel powertrain. Even with a de-carbonised electricity system (100 g CO2/kWh), CO2 emissions are similar to a conventional Diesel fuelled HGV. For the HGV sector, range is key to operator acceptability of new powertrain technologies. This analysis has shown that cumulative benefits of improved electrical powertrains, on-board storage, efficiency improvements and vehicle design in 2025 and 2035 mean that hydrogen and electric fuelled vehicles can be competitive on gravimetric and volumetric density. Overall, the work demonstrates that presently there is no common powertrain solution appropriate for all vehicle types but how subtle improvements at a vehicle component level can have significant impact on the design choices for the wider energy system.
•The operation process of free-piston linear generator is investigated.•The larger the motor force at the starting process, the fewer circulations of the piston reciprocating to meet ignition ...condition.•The “gradually switching strategy” is the best strategy in the intermediate process.•During the generating process, engines indicated power is 2.9kW with an efficiency of 37.3% under medium load.
Free piston linear generator (FPLG) shows unique operation characteristics due to the elimination of crankshaft and connecting rod mechanism. This paper investigates its operation characteristics during each operating process based on the simulation and experiment results. During the starting process, the larger motor force during the starting process, the fewer times of reciprocating pistons which meet the condition of ignition. When the motor force reached 300N, the prototype could adopt one-stroke starting strategy. During the intermediate process, it was found that the “gradually switching strategy” could help to achieve a smoother operation during the intermediate process. And the values of the operation parameters after the intermediate process were lower than those before the intermediate process. During the generating process, cycle-to-cycle variations were observed for piston TDC and in-cylinder gas pressure from the experimental results. According to the experimental results of the FPLG during the generating process, the calculated engine indicated power is 2.9kW, and the corresponding indicated thermal efficiency is 37.3%. Additionally, based on the comparison of the FPLG performance, it is found that the parameters of the FPLG during the generating process are smaller than those when it was operated during the second stage of the starting process, while much higher than those during the first stage of the starting process.
•Different types of system disturbance with specific occurring times were identified.•The influence of each disturbance on the FPE system was characterised.•Technically feasible control variables ...were identified.•Control variable coupled with a system controller design was presented.
In this paper, a fast-response numerical model was used to investigate potential disturbances to a free-piston engine generator (FPEG), i.e. engine cycle-to-cycle variations, misfire and immediate electric load change. During the engine operation, there could be one disturbance taking place or several disturbances take place simultaneously. By identifying different types of system disturbance with specific occurring times, the influence on the system was characterised. It was found that a step change of electric load would induce a corresponding top dead centre (TDC) step change. Low variations on piston TDC are observed when cycle-to-cycle variations take place. When unsuccessful ignition occurs, the engine will stop after one oscillation cycle. Reducing the electric load after misfire would cause more oscillation cycles and require a restart of the engine. Technically feasible control variables were identified and coupled with a PI feedback controller design to minimise the impact of each kind of disturbance, a design which could be used in future FPE control system designs. The controller performance was seen to be satisfactory for the electric load step change, and the piston TDC was controlled to back to the set point in 0.5s.
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